In general, acrylonitrile-butadiene-styrene copolymer (ABS) resins have been widely used in various applications such as vehicles, electrical and electronic components, office equipment, home appliances, toys, office supplies, and the like, due to impact resistance of butadiene, moldability and colorability of styrene, and physical properties such as rigidity, chemical resistance, and the like, and an aesthetic appearance property of acrylonitrile.
As a general method of preparing the ABS resin, a kneading and extruding process (a compounding process) preparing the ABS resin has been used, wherein the compounding process includes preparing grafted ABS (g-ABS) obtained by emulsion polymerization of butadiene and addition of styrene and acrylonitrile thereto, and kneading and extruding the obtained g-ABS together with a styrene-acrylonitrile copolymer (SAN) resin in an extruder.
In addition, a bulk-suspension process preparing the ABS resin by bulk-polymerizing butadiene, styrene, and acrylonitrile at the first stage, and after a phase-conversion step, changing the bulk-polymerized mixture into a suspension-polymerization state, and a mass-continuous process preparing the ABS resin by feeding all monomer raw materials at the first stage, followed by bulk-polymerization have been widely known and used.
The bulk-suspension process has advantages in that since only one reactor is used, the prepared resin has stable physical properties and operation cost and energy consumption are low; however, the bulk-suspension process has disadvantages in that it is not suitable for mass-production.
In addition, the mass-continuous process has advantages in that it is suitable for mass-production; however, the mass-continuous process has disadvantages in that it is not easy to control physical properties of an impact reinforcing agent used in the resin, and as a result, it has a limitation in manufacturing various products.
A kneading and extruding process which is the most widely used until recently uses a relatively simple manufacturing apparatus, and uses g-ABS particles having a small average particle size prepared by a batch type emulsion polymerization, such that a high glossy ABS resin is capable of being prepared and a mixing ratio of raw materials in a kneading and extruding step may be easily changed, thereby being advantageous for controlling the physical properties of the ABS resin. However, when the g-ABS particles are prepared by the emulsion polymerization, an agglomerating process, a washing process, a dewatering process, a drying process, and the like, should be performed, and then a feeding process into an extruder is also needed. In this case, excessive energy is consumed in the dewatering and drying processes, and the g-ABS particles tend to agglomerate, which form a gel spot, causing a problem in that the ABS resin has a poor appearance. In addition, in order to pulverize and disperse the agglomerated g-ABS particles, an extruder having a screw with a strong kneading element is used; however, in this case, discoloration of the ABS resin badly occurs.
Korean Patent No. 0371072 and Korean Patent No. 0371887 disclose a preparation method of an impact reinforced thermoplastic resin composition including supplying an impact reinforcing elastomer containing moisture to a twin-screw extruder, removing the moisture by a compression unit, and mixing with a matrix thermoplastic resin. However, in this case, loss of raw materials occurred while the moisture was removed by the compression unit, and additional processes such as heating, and the like, were required, such that energy consumption was remarkably increased. Accordingly, there were problems that overall preparation cost is increased, and appearance property may be deteriorated and discoloration may be increased due to the increase in the energy consumption of the extruder.
Therefore, there is a need for a method of preparing a thermoplastic resin composition having improved appearance, excellent discoloration resistance to improve color appearance, and a low gel content, that can be prepared with reduced load on an extruder motor to reduce energy consumption.